Currently, in the manufacture of flat sheet products, such as paper, plastic films and textiles, measuring devices in production processes enable the feedback of information used in process control systems, based on measured parameters. Manual or automated process control systems may use this information. Sensors used in such process control systems may need to develop accurate measurement information on a quickly moving, fluttering web, while operating in a high humidity, dirty, hot and/or wet environment. Such sensors usually mount on measurement platforms that scan the sensors slowly in a cross-process direction as the process web moves relatively rapidly in a process direction.
Infrared spectroscopic sensors are common measuring devices for such control systems. These sensors measure the absorption of infrared radiation at specific wavelength bands, indicating a specific property's presence and/or magnitude. Specific characteristics that the sensors may measure include properties such as water, polymers, cellulose and other components of a product. A common application is the measurement of the fraction of water by weight (percent moisture) in a moving paper web during manufacturing.
The infrared spectroscopic sensor measurements utilize the differential absorption of various wavelength bands in the near infrared region, generally 0.75 μm-10.0 μm, by water and other components of the product. Process controllers compare measurements of the transmission and/or reflection of infrared energy at one or more reference wavelengths to measurements of the transmission and/or reflection at one or more absorption wavelengths. The reference wavelengths are selected for a relatively low absorption coefficient by as many of the components of the product as possible, and the absorption wavelengths are selected for a relatively high absorption coefficient. A number of different wavelength measurements may be used to determine and/or reject other interfering parameters, such as the mean optical path length through the product as a result of optical scattering. The sensors generally require the measurement of infrared energy in several spectral bands, all made simultaneously and representing the same area on the moving process. Simultaneous measurement generally requires multiple detectors, each detecting infrared energy at one of the spectral bands of interest. Since the properties of the web that affect the various infrared wavelengths can vary over short distances on the web, any differences in the web areas presented to the various detectors will result in a measurement error. Simply placing the individual detectors proximate to each other is generally inadequate to meet accuracy requirements. The signals from these detectors may be mathematically combined to develop the measurement of interest.
Conventionally, measuring systems have used lead salt detectors such as lead sulfide (PbS) or lead selenide (PbSe) detectors, however measuring systems using Indium Gallium Arsenide (InGaAs) sensors may overcome some of the complexities encountered in using lead salt detectors. InGaAs detectors are commonly used in fiber optic communications realm, and it is known to use infrared light-emitting diodes with an InGaAs device for measuring moisture value of a product. See U.S. Pat. No. 5,870,926. In contrast to lead salt detectors, InGaAs sensors are photovoltaic, so the absorption of light results in a change in voltage rather than a change of resistance as in lead salt detectors. Although both lead salt and InGaAs detectors provide good performance near room temperature, around 27 degrees C., temperature sensitivity of lead salt sensors may require maintenance of a stable sensor temperature within 0.001 degrees C. tolerance to achieve desired measurement accuracy. Since lead salt sensors are so sensitive to temperature, systems using multiple lead salt detectors must perform frequent “standardization” to correct errors produced by temperature drift and sensor dark current. Therefore, some lead salt sensors use only a single detector so the effect of temperature drift or dark current is common to all of the measured wavelengths and can be cancelled out during signal processing. This use of a single detector requires that various wavelengths are measured separately in time, possibly introducing error. InGaAs sensors have an extremely low sensitivity to temperature, less than 1/3000 that of lead salt detectors. Another known difficulty of lead salt sensors is the relatively long time constants, in the millisecond range, which limit the rate of measurement.
Existing sensors that utilize multiple detectors may use an optical beam splitter in attempts to present the same web area to all detectors. A common type of beam splitter is a partially or selectively reflecting mirror. The partial reflector transmits some fraction of the incident energy and reflects most of the remainder. If aligned properly, detectors exposed to the reflected fraction and to the transmitted fraction will “see” the same area of the web. A disadvantage of partial reflecting beam splitters is that the reduction of signal amplitude at each beam splitter limits the practical number of detectors. The selective reflecting beam splitter transmits certain wavelengths and reflects others, rather than dividing the input beam as a fraction of its amplitude, they do so according to its spectral distribution. Practical wavelength-selective beamsplitters may approach 80% efficiency, whereas partial reflective beamsplitters rarely exceed 45% efficiency. Wavelength-selective beamsplitters are more expensive than are partial reflectors and are quite sensitive to the incident angle of the optical energy, working only over a small range of angles about their design incident angle. Reflecting beam splitters also require precise positioning of all detectors.
Accordingly, there is a need for an infrared spectroscopic measuring device that overcomes or lessens these problems.